# std::is_bind_expression

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< cpp‎ | utility‎ | functional

Defined in header `<functional>` |  |
template< class T > struct is_bind_expression; |  | (since C++11)

If `T` is a type produced by a call to std::bind (but not std::bind_front or std::bind_back), this template is derived from std::true_type. For any other type (unless user-specialized), this template is derived from std::false_type.

A program may specialize this template for a program-defined type `T` to implement UnaryTypeTrait with base characteristic of std::true_type to indicate that `T` should be treated by std::bind as if it were the type of a bind subexpression: when a bind-generated function object is invoked, a bound argument of this type will be invoked as a function object and will be given all the unbound arguments passed to the bind-generated object.

### Helper variable template

template< class T > constexpr bool is_bind_expression_v = is_bind_expression<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a function object generated by std::bind, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Example

Run this code

    #include <functional>
    #include <iostream>
    #include <type_traits>

    struct MyBind
    {
        typedef int result_type;
        int operator()(int a, int b) const { return a + b; }
    };

    namespace std
    {
        template<>
        struct is_bind_expression<MyBind> : public true_type {};
    }

    int f(int n1, int n2)
    {
        return n1 + n2;
    }

    int main()
    {
        // as if bind(f, bind(MyBind(), _1, _2), 2)
        auto b = std::bind(f, MyBind(), 2);

        std::cout << "Adding 2 to the sum of 10 and 11 gives " << b(10, 11) << '\n';
    }

Output:

    Adding 2 to the sum of 10 and 11 gives 23

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2010 | C++11 | program-defined specializations could only be derived from std::false_type | can be derived from std::true_type

### See also

bind (C++11) | binds one or more arguments to a function object (function template)

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